The present invention relates to a radio environment analysis apparatus and, more particularly, to a radio environment analysis apparatus for measuring the arrival angle, delay time, and relative power of a signal incoming to the antenna in a multipath environment such as an urban or indoor space.
To realize high-speed digital communication, frequency resources must be effectively used. Many studies have been made for techniques such as a method using the milliwave band where a wide band is essentially ensured from a specific band, a method of reducing a repetitively used frequency with a decrease in cell area, and dynamic zone control effectively using adjacent cell base stations in accordance with the number of users.
As the communication speed increases, interference between multipath codes occurs to decrease the communication quality in an environment such as an urban or indoor space. When the cell area decreases, interference in the same channel occurs.
One of effective means for removing interference between multipath codes and interference in the same channel is to introduce an adaptive antenna effective for removing a long delayed wave and an adaptive equalizer effective for removing a short delayed wave. In this case, to effectively operate the adaptive antenna and adaptive equalizer, the arrival angle and delay time of a signal incoming to the antenna must be measured. Also in dynamic zone control, particularly the arrival angle of a signal incoming to the antenna must be measured for beam control of the antenna.
Known examples of an algorithm of measuring the arrival angle, delay time, and relative power of a signal incoming to the antenna in the multipath environment are a 2D-MUSIC method, FFT-MUSIC method, and 2D-Unitary ESPRIT method.
Evaluation of the indoor propagation environment by the 2D-MUSIC method is reported in "High-Resolution Analysis of Indoor Multipath Propagation Structure", IEICE TRANS. COMMUN., VOL. E78-B, NO. 11, pp. 1450-1457, NOVEMBER 1995 (reference 1).
The principle of the FFT-MUSIC method is described in "Estimation of Propagation Delay Time and Direction of Arrival of Indoor Quasi-Millimeter Multipath Waves Using FFT-MUSIC with Triangular Antenna Array", TECHNICAL REPORT OF IEICE, A.cndot.P95-120, pp. 79-84, February 1996 (reference 2).
Evaluation of propagation characteristics in the urban space using the 2D-Unitary ESPRIT method is described in "High-Resolution 3-D Direction-of-Arrival Determination for Urban Mobile Radio", IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 45, NO. 4, APRIL 1997 (reference 3).
The principle of simultaneously estimating a combination of the arrival angle and delay time of a signal incoming to the antenna is described in "Paired Estimation of Propagation Delay Times and Directions of Arrival of Multipath Waves Using 2D Unitary ESPRIT", TECHNICAL REPORT OF IEICE, A.cndot.P97-78, JULY 1997 (reference 4).
However, these conventional methods of estimating the arrival angle and delay time of a signal incoming to the antenna suffer the following problems.
First, the 2D-MUSIC method in reference 1 and the 2D-Unitary ESPRIT method in reference 4 can estimate only the azimuth plane of the arrival angle and cannot estimate the elevation plane. At present, since a mobile telephone base station in operation controls the elevation plane directivity of the antenna in accordance with the situation, the azimuth and elevation planes of the arrival angle must be simultaneously grasped.
Second, the position of a reception antenna element is mechanically moved for measurement. This is also the same in the 2D-Unitary ESPRIT method in reference 3. The mechanical movement of the reception antenna increases the measurement time. Considering mobile communication, the antenna and equalizer must be controlled in real time, so these methods are difficult to apply to mobile communication.
Third, since a reception signal is normalized using a transmission signal as a reference, the transmission signal must be supplied to the receiver via a cable. Connecting communication stations by the cable essentially contradicts mobile communication.
To the contrary, the FFT-MUSIC method in reference 2 can simultaneously estimate the arrival angles of azimuth and elevation planes and the delay time without requiring any mechanical movement of the reception antenna and any connection cable between communication stations. However, this method does not resolve the arrival angle by a superresolution technique, resulting in low angle resolution and low precision. In reference 3, although the arrival angles of elevation and azimuth planes are estimated by the 2D-Unitary ESPRIT method, the delay time is estimated using a conventional channel counter, resulting in low time resolution and low precision.